Single-ended push-pull amplifier circuit

ABSTRACT

A first amplifying transistor is connected between a first supply terminal and an output terminal, the latter being grounded via a loudspeaker system, and another such transistor between a second supply terminal and the output terminal. A first and a second drive transistor are Darlington connected respectively to the first and the second amplifying transistor. A first biasing circuit is connected between the first supply terminal and the base of the first drive transistor, and a second biasing transistor between the second supply terminal and the base of the second drive transistor. In order for an idling current of stable magnitude to flow immediately when the amplifier circuit is powered on, without need for preadjustment of circuit elements, an improved third biasing circuit is connected between the bases of the two drive transistors. The third biasing circuit comprises two biasing transistors and a serial circuit of two resistors. One of the biasing transistors has its collector connected to the base, and its base to the emitter, of either of the two drive transistors, thereby forming a negative feedback circuit.

BACKGROUND OF THE INVENTION

[0001] This invention relates to amplifiers, to audio amplifiers, and particularly to a single-ended push-pull amplifier circuit suitable for use at the final stage of multistage audio amplifiers.

[0002] Complementary single-ended push-pull amplifiers (FIGS. 1 and 2) have been known and used extensively for the final-stage power amplification of audio signals. Japanese Unexamined Patent Publication No. 55-35520 suggests improvements relating to amplifier circuits of this broad category.

[0003] The typical push-pull amplifier of the kind under consideration has two amplifying transistors, one connected between a positive supply terminal and an output terminal, and the other between a negative supply terminal and the output terminal. The output terminal is grounded via a loudspeaker system. Two other transistors are employed for driving the amplifying transistors. The drive transistors have their base current controlled according to the incoming audio signal, thereby controlling current flow through the amplifying transistors. Thus is the loudspeaker system supplied with the current amplified according to the magnitude of the input signal.

[0004] For operation as a class A amplifier, the biasing circuits incorporated into the amplifier circuit are so preset that idling current flows through the transistors at all times, even when the signal input is zero. Unless kept under constant control, however, the idling current could cause the overheating and eventual breakdown of the transistors. The idling current should not be so great in magnitude as to lead to the transistor overheating but so low as to cause switching distortion, as in class B amplifiers.

[0005] Various suggestions have been made, and some put into practice, for precluding the thermal breakdown of the transistors. Some such known suggestions, bearing particular pertinence to the instant invention, are unsatisfactory by reason of too delicate preadjustment of some circuit elements that must be made at the cost of considerable time and labor to provide idling current of a desired level. Another objection is the prolonged period of time required by the prior art amplifiers of the type in question for the idling current to rise to the required level after the amplifier is switched on.

SUMMARY OF THE INVENTION

[0006] The present invention seeks to make totally unnecessary the time-consuming preadjustment conventionally needed by the class A amplifiers of the type defined to assure the flow of constant-level idling current in the face of temperature variations of the relevant circuit elements such as transistors.

[0007] The invention also seeks to provide for the stabilization of the idling current in the amplifier circuit just after it is switched on, virtually eliminating the warm-up period heretofore required for the idling current to gain its steady-state magnitude.

[0008] Briefly, the present invention may be summarized as a single-ended push-pull amplifier circuit suitable for power amplification of audio signals, among other applications. Included are first current control means (e.g. two transistors in Darlington connection) connected between a first supply terminal and an output terminal for controlling current flow therebetween, and second current control means (e.g. two other transistors in Darlington connection) connected between a second supply terminal and the output terminal for controlling current flow therebetween. A biasing circuit is provided according to the invention which comprises a serial connection of a first and a second resistor, the latter being connected to a control terminal of either of the first and the second current control means, and a first and a second bias transistor. The first bias transistor is connected between a control terminal of the other of the first and the second current control means and the first resistor, and has a control terminal connected to said other of the first and the second current control means. The second bias transistor is connected between the control terminals of the first and the second current control means and has a control terminal connected to a junction between the first and the second resistor.

[0009] Such being the improved construction of the amplifier circuit, particularly the biasing circuit, according to the invention, the current through the bias transistors increases in magnitude with the current through the current control means, resulting in negative feedback of the control voltages for the current control means. The current through the current control means is therefore stabilized without waiting for the warming of the bias transistors.

[0010] Another pronounced advantage of the improved biasing circuit is that the magnitude of the idling current becomes independent of temperatures merely as the two resistors connected in series with the first bias transistor are made equal in value. The resistors can be fixed at the same value. No readjustment whatever prior to shipment, and of course in use, is necessary for provision of an idling current of desired magnitude.

[0011] In an alternative embodiment of the invention a diode is connected in series with the resistors for additional temperature compensation. Various other embodiments will be disclosed to show various forms the amplifier circuit according to the invention can take in practice.

[0012] The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic electrical diagram of a prior art single-ended push-pull amplifier circuit bearing particular pertinence to the instant invention;

[0014]FIG. 2 is a schematic electrical diagram of another prior art push-pull amplifier circuit associated with the FIG. 1 amplifier circuit;

[0015]FIG. 3 is a schematic electrical diagram of a preferred form of single-ended push-pull amplifier circuit according to the invention;

[0016]FIG. 4 is a schematic electrical diagram of another preferred form of single-ended push-pull amplifier circuit according to the invention;

[0017]FIG. 5 is a schematic electrical diagram of yet another preferred form of single-ended push-pull amplifier circuit according to the invention;

[0018]FIG. 6 is a schematic electrical diagram of still another preferred form of single-ended push-pull amplifier circuit according to the invention;

[0019]FIG. 7 is a schematic electrical diagram of yet another preferred form of single-ended push-pull amplifier circuit according to the invention;

[0020]FIG. 8 is a schematic electrical diagram of a further preferred form of single-ended push-pull amplifier circuit according to the invention;

[0021]FIG. 9 is a schematic electrical diagram of a still further preferred form of single-ended push-pull amplifier circuit according to the invention;

[0022]FIG. 10 is a schematic electrical diagram of a yet further preferred form of single-ended push-pull amplifier circuit according to the invention; and

[0023]FIG. 11 is a schematic electrical diagram of an emitter-follower amplifier circuit to which are also applicable the concepts of the invention.

DETAILED DESCRIPTION

[0024] It is considered essential that the prior art complementary single-ended push-pull amplifier circuit be shown and described in some more detail, the better to make clear the features and advantages of the instant invention. With reference to FIG. 1 the prior art amplifier circuit comprises a first supply terminal 1 for providing a positive supply voltage +Vcc, a second supply terminal 2 for providing a negative supply voltage −Vcc, an output terminal 3, a ground terminal 4, four final-stage amplifier elements shown as transistors Q₁, Q₂, Q₃ and Q₄, and four biasing circuits 5, 6, 7 and 8. A load 9 such as a loudspeaker system is connected between output terminal 3 and ground terminal 4. It is understood that the load 9 has no output transformer.

[0025] The first amplifier element Q₁ is a npn -transistor having a collector connected to the first supply terminal 1, and an emitter connected to the output terminal 3 via a resistor R₁. This resistor has a resistance value as low as, say, 0.3 ohm. The second amplifier element Q₂ is an pnp—transistor having a collector connected to the second supply terminal 2, and an emitter connected to the output terminal 3 via another resistor R₂. This second resistor has the same resistance value as that of the first resistor R₂.

[0026] The third transistor Q₃, an npn-transistor, is for driving the first transistor Q₁, having a collector connected to the collector of the first transistor, and an emitter connected to the base of the first transistor to provide a Darlington amplifier circuit. The fourth transistor Q₄ is of the pnp type for driving the second transistor Q₂, having a collector connected to that of the second transistor, and an emitter connected to the base of the second transistor to provide another Darlington amplifier circuit.

[0027] The first biasing circuit 5 has a resistor R_(a) connected between the first supply terminal 1 and the base of the third transistor Q₃.

[0028] The second biasing circuit 6, which might be termed a signal input circuit, comprises an npn-transistor Q_(a) and three resistors R_(b), R_(c) and R_(d). The transistor Q_(a) has a collector connected to the base of the fourth transistor Q₄, and an emitter connected to the second supply terminal 2 via the resistor R_(b). The resistor R_(c) is connected between the ground and the base of the transistor Q_(a). The resistor R_(d) is connected between the base of the transistor Q_(a) and the second supply terminal 2. The base of the transistor Q_(a) is grounded via a coupling capacitor C₁ and a source e₁ of an input audio signal to be amplified.

[0029] The third biasing circuit 7 comprises a resistor R_(e) and a serial circuit of three temperature-compensating diodes D₁, D₂ and D₃ connected in series therewith. The fourth biasing circuit 8 takes the form of a resistor R_(f) connected between the bases of the first transistor Q₁ and second transistor Q₂.

[0030] As is well known, when supplied with the audio signal from its source e₁, the transistor Q_(a) of the second biasing circuit 6 will change in impedance with the input current magnitude, thereby controlling the base currents of the driving transistors Q₃ and Q₄ and hence the currents through the amplifying transistors Q₁ and Q₂. Thus will the load 9 be supplied with a current that varies in power level with the input current. As the input current reverses in direction, the load 9 will be energized in two opposite directions, one through the path comprising the first supply terminal 1, first amplifying transistor Q₁, first resistor R₁, and load 9, and the other through the path comprising the ground terminal 4, load 9, second resistor R₂, second amplifying transistor Q₂, and second supply terminal 2.

[0031] The biasing circuits 5-7 are so made that so-called “idling current” flows through the transistors Q₁-Q₄ when the input audio signal is zero, that is, when no signal current is flowing through the load 9. This idling current is expressed as:

I _(id) =[V _(BB1)−(V _(BE1) +V _(BE2) +V _(BE3) +V _(BE4))]/(R ₁ +R ₂)

[0032] where

[0033] I_(id)=idling current

[0034] V_(BB) voltage across the third biasing circuit 7 when the input signal is zero

[0035] V_(BE1)=base-emitter voltage of the first transistor Q₁

[0036] V_(BE2)=base-emitter voltage of the first transistor Q₂

[0037] V_(BE3)=base-emitter voltage of the first transistor Q₃

[0038] V_(BE4)=base-emitter voltage of the first transistor Q₄.

[0039] Much less in magnitude than the peak value of the input signal, the idling current is known to be effective for reduction of the switching distortion that occurs in class B amplifiers when the voltage is approximately zero.

[0040] Let us assume that the bias voltage V_(BB) due to the third biasing circuit 7 were constant regardless of temperatures. Since the transistors Q₁-Q₄ have a temperature coefficient of approximately −2 millivolt per degree in centigrade, the idling current id would increase in magnitude with the temperature of these transistors. The greater idling current would in turn give rise to greater power loss due to the transistors Q₁-Q₄, elevating their temperatures. The result would be an even more rise in the idling current. The cycle might repeat itself indefinitely until the transistors broke down in the worst case.

[0041] The temperature-compensating diodes D₁-D₃ of the third biasing circuit 7 are designed to preclude the thermal breakdown of the transistors Q₁-Q₄. Thermally coupled to these transistors, the diodes D₁-D₃ have approximately the same temperature coefficient as that of the transistors. Consequently, with a rise in temperature, the bias voltage V_(BB) will drop to prevent an increase in the idling current level. The biasing circuit 7 with the three temperature-compensating diodes D₁-D₃ is objectionable as it requires fine adjustment of the resistor R_(e) for optimization of the bias voltage V_(BB).

[0042] It may be contemplated to modify the third biasing circuit 7 into the circuit 7 a shown in FIG. 2. The modified biasing circuit 7 a has a transistor Q_(b) and two resistors R_(e1) and R_(e2) connected between the bases of the transistors Q₃ and Q₄. Thermally coupled to the transistors Q₁-Q₄, the biasing transistor Q_(b) has its collector-emitter resistance lowered with a rise in the temperatures of the transistors Q₁-Q₄. The bias voltage V_(BB) due to the biasing circuit 7 a will drop with the total resistance of the biasing circuit 7 a, preventing an increase in idling current I_(id).

[0043] The bias voltage due to the modified biasing circuit 7 a can be expressed as:

V _(BB) =V _(BE1) +V _(BE2) +V _(BE3) +V _(BE4) +I _(id) (R ₁ +R ₂)

V _(BB) V _(BEb)(R _(e1) +R _(e2))/R_(e2)

[0044] where

[0045] V_(BEb)=base-emitter voltage of the transistor Q_(b)

[0046] V_(BB)=bias voltage across the modified biasing circuit 7 a.

[0047] Since V_(BE1), V_(BE2), V_(BE3), V_(BE4) and V_(BEb) are all approximately equal to one another, each of these may be redesignated V_(BE). Then, from the foregoing equations:

V _(BE) (R _(e1) +R _(e2))/R_(e2)=4 V _(BE) +I _(id) (R ₁ +R ₂)

I _(id) (R ₁ +R ₂)=[V _(BE)(R _(e1) +R _(e2) )/R _(e2)]−4V _(BE) V _(BE)(R _(e1)−3R _(e2) )/R _(e2).

[0048] As is clear from the equation just above, the idling current I_(id) can be reduced to zero by making the resistor R_(e1) of the FIG. 2 biasing circuit 7 a thrice as high in resistance value as the other resistor Re₂. Then the idling current will not augment in the face of the temperature rise of the transistors Q₁-Q₄, so that these transistors will be saved from destruction due to the heat cycle explained previously.

[0049] The solution of FIG. 2 is nevertheless unsatisfactory for several reasons. First, either of the resistors R_(e1) and R_(e2) must be semifixed for obtaining an idling current of required magnitude. For, should the values of these resistors be fixed at a ratio of one to three, no idling current would flow, a class B amplifier. It is well known that distortion due to switching occurs in class B amplifiers when the voltage is approximately zero. A constant flow of some idling current is mandatory for distortionless amplification.

[0050] Thus, in the case of the FIG. 2 circuit, the values of the resistors R_(e1) and R_(e2) must be critically determined in relation to each other so that the resulting idling current will cause neither switching distortion nor the thermal breakdown of the transistors. Such critical adjustment of the resistors R_(e1) and R_(e2) led to higher manufacturing costs because of longer periods of time required for that purpose.

[0051] A further objection to the FIG. 2 amplifier circuit is an inconveniently prolonged length of time required for the idling current to gain a steady-state level following the closure of the power switch. Because of the criticality of the idling current magnitude pointed out above, the resistors R_(e1) and R_(e2) were so preadjusted that the steady-state level was obtained after the transistors were heated to an appropriate temperature.

[0052] The problems so far discussed in connection with the prior art are by no means specific to the amplifier circuits of FIGS. 1 and 2. Similar problems arose in amplifier circuits that were connected to the signal sources in different ways, that had different types of push-pull transistors, that had different power supply circuits, or that were of emitter-follower, rather than push-pull, construction.

[0053] The amplifier circuit according to the invention defeats all such shortcomings of the prior art, as will become better understood from the following detailed description of some preferred forms.

EMBODIMENT OF FIG. 3

[0054] In FIG. 3 is shown the present invention as embodied in an audio power amplifier circuit. The representative power amplifier circuit according to the invention features an improved third biasing circuit 7 b in substitution for its FIG. 1 counterpart 7 or FIG. 2 counterpart 7 a. The representative amplifier circuit is akin in the other details of construction to the prior art amplifier circuits of FIGS. 1 and 2, so that like reference characters are used to denote like parts in FIGS. 1-3, as well as in all the other figures to be referred to subsequently.

[0055] The two transistors Q₁ and Q₂ of the representative power amplifier circuit according to the invention may be more broadly envisaged as current control elements, controlling current flow from the supply terminals 1 and 2 to the loudspeaker system 9. Each the first and the second transistors Q₁ and Q₂ has a collector as a first main terminal, a emitter as a second main terminal, and a base as a control terminal. The other two transistors Q₃ and Q₄ are Darlington-connected to the transistors Q₁ and Q₂, respectively, for driving them. The two Darlington-connected transistors Q₁ and Q₃ constitute in combination a first current control or amplifier means. Essentially, this first control means is equivalent to one npn-transistor, controlling current flow from collector to emitter of the current control transistor Q₁ under the control of a signal applied to the base of the drive transistor Q₃.

[0056] Similarly, the other two Darlington-connected transistors Q₂ and Q₄ constitute in combination a second current control or amplifier means essentially equivalent to one pnp-transistor. The second current control means controls current flow from collector to emitter of the current control transistor Q₂ under the control of a signal applied to the base of the drive transistor Q₄.

[0057] Constituting a feature of this invention, the third biasing circuit 7 b comprises two additional transistors Q₅ and Q₆ hereinafter referred to as bias transistors, and two resistors R₃ and R₄, hereinafter referred to as bias resistors. It is understood that the bias transistors Q₅ and Q₆ are both thermally coupled to all or some of the current control transistors Q₁-Q₄ by means of a heat sink, not shown, of conventional make. In any event it is recommended that the bias transistors Q₅ and Q₆ be thermally coupled to at least either of the amplifying transistors Q₁ and Q₂.

[0058] The first bias transistor Q₅ has its collector connected to the base of the first drive transistor Q₃, and its base to the emitter of the first drive transistor. The first bias resistor R₃ has one extremity thereof connected to the emitter of the first bias transistor Q₅, and the other extremity to one extremity of the second bias resistor R₄. The other extremity of this second bias resistor R₄ is connected to the base of the second drive transistor Q₄. The second bias transistor Q₆ has its collector connected to the base of the first drive transistor Q₃, its emitter to the base of the second drive transistor Q₄, and its base to the junction P₁ between the bias resistors R₃ and R₄.

[0059] All but the third biasing circuit 7 b of the FIG. 3 amplifier circuit according to the invention is constructed as previously set forth in connection with the prior art circuits of FIGS. 1 and 2.

[0060] In the practice of the invention the voltages +V_(cc) and −V_(cc) of the supply terminals 1 and 2 may be set at +9.4 and −9.4 volts; the resistors R₁ and R₂ both at 0.22 ohm; the impedance of the speaker system 9 at 4-8 ohms; the resistors R₃ and R₄ both at 1.8 kilohms; and the resistor R_(f) of the fourth biasing circuit 8 at 330 ohms. Preferably, the drive transistors Q₃ and Q₄ should have a transition frequency f_(T) higher than that of the amplifying transistors Q₁ and Q₂.

OPERATION OF THE FIG. 3 EMBODIMENT

[0061] The transistor Q_(a) of the second biasing circuit 6 will change in impedance with the alternating input signal from its source e₁, causing current flow through the transistors Q₂ and Q₄ during the positive half-waves of the input signal and through the transistors Q₁ and Q₃ during the negative half-waves of the input signal. Current will flow in one direction through the path comprising the ground terminal 4, load 9, second resistor R₂, second amplifying transistor Q₂, and second supply terminal 2, and in the other direction through the path comprising the first supply terminal 1, first amplifying transistor Q₁, first resistor R₁, load 9, and ground terminal 4.

[0062] The third biasing circuit 7 b functions to cause the flow of idling current of required magnitude right after the circuit is turned on, without causing the thermal breakdown of the transistors Q₁-Q₄. Base current will flow to the first drive transistor Q₃ via the resistor R_(a) of the first biasing circuit 5 upon energization of the supply terminals 1 and 2. The equivalent resistance of the third biasing circuit 7 b is infinitely great immediately after the circuit is powered on, because then both transistors Q₅ and Q₆ are off. Consequently, the bias voltages impressed between the base and emitter of the drive transistors Q₃ and Q₄ depend upon the resistances of the first, second and fourth biasing circuits 5, 6 and 8.

[0063] Also, because of the noted high resistance of the third biasing circuit 7 b just after the circuit is switched on, the base potential of the first drive transistor Q₃ will be high, and that of the second drive transistor Q₄ low, resulting in the flow of relatively great base current to the these transistors Q₃ and Q₄. Base current will also flow to the amplifying transistors Q₁ and Q₂ via the drive transistors Q₃ and Q₄. These drive transistors Q₃ and Q₄ will then function as bias resistors, so to say, for the amplifying transistors Q₁ and Q₂.

[0064] At the same time, base current will be supplied to the transistor Q₅ of the third biasing circuit 7 b via the first drive transistor Q₃. Thereupon the collector-emitter resistance of the bias transistor Q₅ will drop rapidly, resulting in a rise in the potential at the junction P₁ between the resistors R₃ and R₄. Then, with the flow of base current to the other bias transistor Q₆, its collector-emitter resistance will also drop. Thus will the resistance of the complete third biasing circuit 7 b decrease.

[0065] A drop in the resistance of the third biasing circuit 7 b is tantamount to a drop in the resistance between the bases of the two drive transistors Q₃ and Q₄. The base potential of the first drive transistor Q₃ will therefore drop, causing a decrease in the base current of the first drive transistor Q₃ and in the base current and collector current of the first amplifying transistor Q₁. The base potential of the second drive transistor Q₄ will rise, on the other hand, with a consequent drop in the base current and collector current of the transistors Q₂ and Q₄.

[0066] Thus the third biasing circuit 7 b functions to cause negative feedback of the collector current of the drive transistors Q₃ and Q₄. As a result, as the current flowing through the current control transistors Q₁-Q₄ tends to increase in magnitude with a rise in their temperature, the resistance of the third biasing circuit 7 b will drop before the accompanying temperature rise of the bias transistors Q₅ and Q₆, which are thermally coupled as aforesaid to the current control transistors Q₁-Q₄. The drop in the resistance of the third biasing circuit 7 b will result in the limitation of the current flow through the current control transistors Q₁-Q₄. That is to say that the idling current flowing through the current control transistors will stabilize instantly upon powering of the amplifier circuit.

[0067] Possibly, in the practice of the instant invention, the current control transistors Q₁-Q₄ may rise in temperature due to fluctuations in the performance characteristics of the circuit elements. Even then the current flowing through the current control transistors will be limited by the negative feedback action of the third biasing circuit 7 b , so that the transistors will not heat up to such temperatures as to result in their breakdown.

[0068] The bias voltage across the third biasing circuit 7 b of the FIG. 3 amplifier circuit is defined as:

V _(BB) =V _(BE1) +V _(BE2) +V _(BE3) +V _(BE4) +I _(id) (R ₁ +R ₂)

V _(BB) =V _(BE3) +V _(VE5) +R ₃(V _(BE6) /R ₄)+V_(BE6)

[0069] where

[0070] Vhd BB=bias voltage across the third biasing circuit 7 b

[0071] V_(BE5)=base-emitter voltage of the first bias transistor Q₅

[0072] V_(BE6)=base-emitter voltage of the second bias transistor Q₆.

[0073] The idling current of the FIG. 3 amplifier circuit is defined as:

I _(id) =[V _(BE5) +R ₃(V _(BE6) /R ₄)+V _(BE6)−(V _(BE1) +V _(BE2) +V _(BE4))]/ (R ₁ +R ₂).

[0074] Since V_(BE1), V_(BE2), V_(BE3), V_(BE4), V_(BE5) and V_(BE6) are all approximately equal to one another, let each of these be designated V_(BE). Then the idling current of the FIG. 3 amplifier circuit can be rewritten as:

I _(id) =[R ₃(V _(BE) /R ₄)−V _(BE)]/ (R ₁ +R ₂)=V _(BE)(R₃ −R ₄)/[(R ₁ +R ₂)R ₅].

[0075] If the temperature coefficient of the transistors Q₁-Q₆ is −2 millivolt per degree in centigrade, the temperature coefficient ΔI_(id)/ΔT of the idling current I_(id) is defined as:

ΔI _(id) /ΔT={(R ₃ −R ₄)/ [(R ₁ +R ₂)R ₄]}×(−2mV/·C).

[0076] As is apparent from the equation just above, the idling current lid of the FIG. 3 amplifier circuit becomes independent of temperatures if R₃=R₄, that is, if the two resistors R₃ and R₄ of the third biasing circuit 7 b are equal in resistance value. It is thus seen that, thanks to the novel concepts of this invention, the idling current of proper magnitude can be preset by use of the fixed resistors only and without any adjustment whatever, affording a substantive saving in the manufacturing costs of the class A amplifiers of this kind.

[0077] It has been stated that the bias transistors Q₅ and Q₆ are thermally coupled to at least either of the amplifying transistors Q₁ and Q₂. Therefore, as the current flowing through the amplifying transistors Q₁ and Q₂ tends to increase due to a temperature rise, so will the current flowing through the bias transistors Q₅ and Q₆ thereby restricting the increase of the current flowing through the amplifying transistors. This thermal coupling of the bias transistors Q₅ and Q₆ to either or both of the amplifying transistors Q₁ and Q₂ serves to preclude the thermal breakdown of the amplifying transistors even in the case where the resistors R₃ and R₄ are not exactly set equal to each other in value. In short the amplifying transistors Q₁ and Q₂ is positively saved from thermal breakdown both by the negative feedback action of the third biasing circuit 7 b and by the temperature-compensating action of the bias transistors Q₅ and Q₆.

EMBODIMENT OF FIG. 4

[0078] The third biasing circuit 7 c shown in FIG. 4 differs from its FIG. 3 counterpart 7 b in additionally having a diode D connected between the first bias transistor Q₅ and the first bias resistor R₃. It is understood that like the bias transistors Q₅ and Q₆, the diode D is thermally coupled to at least either of the four current control transistors Q₁-Q₄ and has approximately the same temperature coefficient therewith. The FIG. 4 amplifier circuit is similar to that of FIG. 3 in all the other details of construction.

[0079] Thus, also like the bias transistors Q₅ and Q₆, the diode D functions for temperature compensation of the current control transistors Q₁-Q₆, making the third biasing circuit 7 c even more effective to prevent the thermal breakdown of the current control transistors.

EMBODIMENT OF FIG. 5

[0080] The power amplifier circuit of FIG. 5 features another modified third biasing circuit 7 d and is identical with the FIG. 3 power amplifier circuit in all the other details of construction. The FIG. 5 third biasing circuit 7 d comprises two resistors R₃ and R₄ and two pnp-transistors Q₅ and Q₆. The first bias transistor Q₅ has its collector connected to the base of the second drive transistor Q₄, and its base to the emitter of the second drive transistor. The first bias resistor R₃ has one extremity thereof connected to the emitter of the first bias transistor Q₅, and the other extremity to one extremity of the second bias resistor R₄. The other extremity of this second bias resistor R₄ is connected to the base of the first drive transistor Q₃. The second bias transistor Q₆ has its emitter connected to the base of the first drive transistor Q₃, its collector to the base of the second drive transistor Q₄, and its base to the junction P₁ between the bias resistors R₃ and R₄.

[0081] The FIG. 5 third biasing circuit 7 d is essentially similar in operation to its FIG. 3 counterpart 7 b . Namely, upon decrease in the resistance of the second drive transistor Q₄ with an increase in current flowing through the current control transistors Q₁-Q₄, the current through the first bias transistor Q₅ will increase, and so will the current through the second bias transistor Q₆. As the resistance of the third biasing circuit 7 d decreases, so will the current through the four current control transistors Q₁-Q₄. These transistors will be thus saved from thermal breakdown.

EMBODIMENT OF FIG. 6

[0082] The third biasing circuit 7 e shown in FIG. 6 has a diode D in addition to all the components of the FIG. 5 third biasing circuit 7 d . Connected between the first bias transistor Q₅ and the first bias resistor R₃, the diode D is thermally coupled to at least either of the four current control transistors Q₁-Q₄ and has approximately the same temperature coefficient therewith. The FIG. 6 amplifier circuit is similar to that of FIG. 3 in all the other details of construction.

[0083] Thus, just like FIG. 4 diode, the diode D functions for temperature compensation of the current control transistors Q₁- Q₄ , making the third biasing circuit 7 e even more effective than the FIG. 5 circuit 7 d to prevent the thermal breakdown of the current control transistors.

EMBODIMENT OF FIG. 7

[0084] The FIG. 7 power amplifier circuit differs from the FIG. 3 embodiment in having a second signal source e₂, in addition to the first e₁, which is connected to a first biasing circuit 5 a of correspondingly modified construction via a coupling capacitor C₂. The FIG. 7 amplifier circuit is akin to that of FIG. 3 in all the other details of construction.

[0085] The modified first biasing circuit 5 a has one pnp-transistor Q_(c) and three resistors R_(a), R_(g) and R_(h). The bias transistor Q_(c) has its emitter connected to the first supply terminal 1 via the resistor R_(a), and its collector to the base of the first drive transistor Q₃. The resistor R_(g) is connected between the first supply terminal 1 and the base of the bias transistor Q_(c), The resistor R_(h) is connected between the base of the bias transistor Q_(c) and the ground. For supplying an audio signal in phase with that from the first signal source e₁, the second signal source e₂ is connected to the base of the bias transistor Q_(c) via the second coupling capacitor C₂.

[0086] The biasing circuits 5 a and 6 will operate opposite to each other, causing current flow through the amplifying transistors Q₁ and Q₂ according to the input signals from their sources e₁ and e₂. Identical with the FIG. 3 embodiment in all the other details of construction and operation, the FIG. 7 power amplifier circuit gains the same advantages therewith.

EMBODIMENT OF FIG. 8

[0087] The FIG. 8 embodiment features a fourth biasing circuit 8 a different from its FIG. 3 counterpart 8. The fourth biasing circuit 8 a has two resistors R_(ƒ1) and R_(ƒ2). The resistor R_(ƒ1) is connected between the base of the first amplifying transistor Q₁ and the output terminal 3. The other resistor R_(ƒ2) is connected between the output terminal 3 and the base of the second amplifying transistor Q₂. The FIG. 8 embodiment is similar to that of FIG. 3 in all the other details of construction and, therefore, in operation as well.

EMBODIMENT OF FIG. 9

[0088] The power amplifier circuit of FIG. 9 differs from that of FIG. 3 only in that the amplifying transistors Q₁ and Q₂ of npn and pnp types in the latter are replaced by field-effect transistors Q_(1a) and Q_(1b) of n and p types in the former. The first amplifying field-effect transistor Q_(1a) has its drain connected to the first supply terminal 1, its source to the first resistor R₁, and its gate to the emitter of the first drive transistor Q₃. The second amplifying field-effect transistor Q_(1b) has its drain connected to the second supply terminal 2, its source to the second resistor R₂, and its gate to the emitter of the second drive transistor Q₄.

[0089] Like the FIG. 3 transistors Q₁ and Q₂, the field-effect transistors Q_(1a) and Q_(1b) are controlled by the drive transistors Q₃ and Q₄, respectively; that is, the resistance values of the field-effect transistors depend upon those of the drive transistors. The FIG. 9 amplifier circuit therefore operates in the same manner as does the FIG. 3 circuit and gains the same advantages therewith.

EMBODIMENT OF FIG. 10

[0090]FIG. 10 shows an adaptation of this invention to a unitary-power-supply construction, with the second supply terminal 2 a grounded, and with a capacitor C connected between the output terminal 3 and the load 9. Even though the second supply terminal 2 a is grounded, alternating current can be supplied to the load 9 as in all the foregoing embodiments thanks to the provision of the capacitor C, so that this embodiment gains the same advantages as does that of FIG. 3.

[0091] Incidentally, the illustrated connection of the resistor R_(c) between the collector and base of the transistor Q_(a) of the second biasing circuit 6 is not essential; instead, it could be connected between the output terminal 3 and the base of the transistor Q_(a).

EMBODIMENT OF FIG. 11

[0092] In FIG. 11 is shown the invention as adapted for an emitter follower amplifier circuit. This embodiment employs but one current control or amplifying element in the form of an npn-transistor Q₁₁, which has its collector connected to the first supply terminal 1, and its emitter to the grounded second supply terminal 2 a via the output terminal 3 and a first resistor R₁₁. The load 9 is connected in parallel with the first resistor R₁₁ via a capacitor C.

[0093] For driving the amplifying transistor Q₁₁ there is provided another npn-transistor Q₁₂ which has its collector connected to the first supply terminal 1, and its emitter to the base of the current control transistor. Thus, like the transistors Q₁ and Q₃ of the FIG. 3 amplifier circuit, the transistors Q₁₁ and Q₁₂ are Darlington connected, constituting a current control means or, equivalently, a single npn-transistor.

[0094] Constituting a first biasing circuit 5 b, a resistor R₁₂ is connected between the first supply terminal 1 and the base of the drive transistor Q₁₂. A second biasing circuit 7ƒ comprises two transistors Q₁₃ and Q₁₄ and two resistors R₁₃ and R₁₄. Their connections are the same as those of their counterparts in the FIG. 3 third biasing circuit 7 b , except that the resistor R₁₄ and transistor Q₁₄ are both connected to the grounded second supply terminal 2 a.

[0095] A further resistor R₁₅ is connected between the base of the amplifying transistor Q₁₁ and the grounded second supply terminal 2 a. A further npn-transistor Q₁₅ has its collector connected to the base of the drive transistor Q₁₂, and its emitter to the grounded supply terminal 2 a via a resistor R₁₆. A resistor R₁₇ is connected between the first supply terminal 1 and the base of the transistor Q₁₅ and a resistor R₁₈ between the base of the transistor Q₁₅ and the grounded supply terminal 2 a. The signal source e₁ is connected to the base of the transistor Q₁₅ via the coupling capacitor C₁.

[0096] The transistors Q₁₁, Q₁₂, Q₁₃, Q₁₄ and Q₁₅ of the FIG. 11 embodiment are equivalent in function to the transistors Q₁, Q₃, Q₅, Q₆ and Q_(a), respectively, of the FIG. 3 embodiment. Similarly, the resistors R₁₁, R₁₂, R₁₃, R₁₄, R₁₅, R₁₆ and R₁₇ of the FIG. 11 embodiment are equivalent in function to the resistors R₁, R_(a), R₃, R₄, R_(ƒ), R_(b), R_(c) and R_(d), respectively, of the FIG. 3 embodiment. Consequently, the biasing circuit 7ƒ functions to make negative feedback control of the current through the amplifying transistor Q₁₁ and hence to protect the same from thermal breakdown.

POSSIBLE MODIFICATIONS

[0097] Notwithstanding the foregoing detailed disclosure, it is not desired that the present invention be limited by the exact showing of the drawings or the description thereof. The following, then, is a brief list of possible modifications or alterations of the illustrated embodiments which are all considered to fall within the scope of the invention:

[0098] 1. The FIGS. 4-6, and 8-10 embodiments could all be adapted for use with two signal sources e₁ and e₂ as in FIG. 7 embodiment.

[0099] 2. The fourth biasing circuit 8 of the FIGS. 4-7, 9 and 10 embodiments could be replaced by its FIG. 8 counterpart 8 a.

[0100] 3. The resistor R₇₁ of the FIGS. 4-9 embodiments could be connected as in the FIG. 10 embodiment.

[0101] 4. The amplifying transistors Q₁ and Q₂ of the FIGS. 4-8 and 10 embodiments, as well as the amplifying transistor Q₁₁ of the FIG. 11 embodiment, could all be replaced by field-effect transistors as in the FIG. 9 embodiment.

[0102] 5. The unitary power supply system of FIG. 10 could be applied to all of the FIGS. 4-9 embodiments.

[0103] 6. Various other known methods of signal inputting could be employed.

[0104] 7. As indicated by the broken lines designated 20 in FIGS. 3, 4 and 7-10, the bias transistor Q₅ could have its base connected to the emitter, or source, of the first amplifying transistor Q₁ or Q_(1a), instead of to the emitter of the first drive transistor Q₃. Similarly, as indicated by the broken lines 20 in FIGS. 5 and 6, the transistor Q₅ could have its base connected to the emitter of the second amplifying transistor Q₂, instead of to the emitter of the second drive transistor Q₄. In these cases the drive transistors Q₃ and Q₄ might be omitted, with the bases of the final-stage transistors connected to the biasing circuit 5 or 5 a.

[0105] 8. In the FIG. 11 embodiment, too, the transistor Q₁₃ of the biasing circuit 7ƒ could have its base connected to the emitter of the amplifying transistor Q₁₁, as indicated by the broken line 20, instead of to the emitter of the drive transistor Q₁₂. In this case the drive transistor Q₁₂ might be omitted, with the base of the final-stage transistor connected to the biasing circuit 5 b.

[0106] 9. The FIG. 5 biasing circuit 7 d could be employed in the FIGS. 7-10 embodiments.

[0107] 10. As indicated by the broken lines in FIGS. 7-11, a diode D could be connected in series with the resistor R₃ or R₁₃.

[0108] 11. Darlington connections of three or more transistors could be used for current amplification in the embodiment disclosed above, with the emitter of each preselected amplifier connected to the base of the bias transistor Q₅ or Q₁₃. 

What is claimed is:
 1. A single-ended push-pull amplifier circuit suitable for power amplification of audio signals, among other applications, the amplifier circuit comprising: (a) a first and a second supply terminal; (b) an output terminal to be connected to a load; (c) first current control means connected between the first supply terminal and the output terminal for controlling current flow therebetween, the first current control means having a control terminal; (d) second current control means connected between the second supply terminal and the output terminal for controlling current flow therebetween, the second current control means having a control terminal; (e) a serial connection of a first and a second resistor, the second resistor being connected to the control terminal of either of the first and the second current control means; (f) a first bias transistor connected between the control terminal of the other of the first and the second current control means and the first resistor, the first bias transistor having a control terminal connected to said other of the first and the second current control means; and (g) a second bias transistor connected between the control terminals of the first and the second current control means, the second bias transistor having a control terminal connected to a junction between the first and the second resistor.
 2. The amplifier circuit of claim 1 further comprising a diode connected between the first resistor and the first bias transistor.
 3. The amplifier circuit of claim 1 wherein the first and the second supply terminal are for providing voltages that are opposite in polarity and equal in absolute value.
 4. The amplifier circuit of claim 1 wherein one of the first and the second supply terminal is grounded, wherein the amplifier circuit further comprises a coupling capacitor connected to the output terminal, and wherein the load is to be connected between the grounded supply terminal and the coupling capacitor.
 5. The amplifier circuit of claim 1 wherein the first current control means comprises: (a) a first current control element having a first main terminal connected to the first supply terminal, a second main terminal connected to the output terminal, and a control terminal; and (b) a first drive transistor having a collector connected to the first supply terminal, a emitter connected to the control terminal of the first current control element, and a base as the control terminal of the first current control means; and wherein the second current control means comprises: (a) a second current control element having a first main terminal connected to the second supply terminal, a second main terminal connected to the output terminal, and a control terminal; and (b) a second drive transistor having a collector connected to the second supply terminal, a emitter connected to the control terminal of the second current control element, and a base as the control terminal of the second current control means.
 6. The amplifier circuit of claim 5 wherein the second resistor is connected to the base of the second drive transistor, the collector of the first bias transistor is connected to the base of the first drive transistor, the emitter of the first bias transistor is connected to the first resistor, and the base of the first bias transistor is connected to the emitter of the first drive transistor.
 7. The amplifier circuit of claim 5 wherein the second resistor is connected to the base of the second drive transistor, the collector of the first bias transistor is connected to the base of the first drive transistor, the emitter of the first bias transistor is connected to the first resistor, and the base of the first bias transistor is connected to the second main terminal of the first current control element.
 8. The amplifier circuit of claim 5 wherein the second resistor is connected to the base of the first drive transistor, the collector of the first bias transistor is connected to the base of the second drive transistor, the emitter of the first bias transistor is connected to the first resistor, and the base of the first bias transistor is connected the emitter of the second drive transistor.
 9. The amplifier circuit of claim 5 wherein the second resistor is connected to the base of the first drive transistor, the collector of the first bias transistor is connected to the base of the second drive transistor, the emitter of the first bias transistor is connected to the first resistor, and the base of the first bias transistor is connected to the second main terminal of second current control element.
 10. The amplifier circuit of claim 1 further comprising: (a) a third resistor connected between the first current control means and the output terminal; and (b) a fourth resistor connected between the second current control means and the output terminal.
 11. An amplifier circuit comprising: (a) a supply terminal; (b) a grounded terminal; (c) an output terminal to be connected to the grounded terminal via a load; (d) current control means connected between the supply terminal and the output terminal for controlling current flow therebetween, the first current control means having a control terminal; (e) a first resistor connected between the output terminal and the grounded terminal; (f) a biasing circuit connected between the supply terminal and the control terminal of the current control means; (g) a serial connection of a second and a third resistor, the third resistor being connected to the grounded terminal; (h) a first bias transistor connected between the control terminal of the current control means and the second resistor, the first bias transistor having a control terminal connected to the current control means; and (i) a second bias transistor connected between the control terminal of the current control means and the grounded terminal, the second bias transistor having a control terminal connected to a junction between the second and the third resistor.
 12. The amplifier circuit of claim 11 wherein the current control means comprises: (a) a current control element having a first main terminal connected to the first supply terminal, a second main terminal connected to the output terminal, and a control terminal; and (b) a drive transistor having a collector connected to the first supply terminal, a emitter connected to the control terminal of the current control element, and a base as the control terminal of the current control means.
 13. The amplifier circuit of claim 12 wherein the third resistor is connected to the grounded terminal, the collector of the bias transistor is connected to the base of the drive transistor, the emitter of the first bias transistor is connected to the second resistor, and the base of the first bias transistor is connected to the emitter of the drive transistor.
 14. The amplifier circuit of claim 12 wherein the third resistor is connected to the grounded terminal, the collector of the first bias transistor is connected to the base of the drive transistor, the emitter of the first bias transistor is connected to the second resistor, and the base of the first bias transistor is connected to the second main terminal of the current control element. 